Frame Flex on GCN

[Mark McM]

Originally Posted by kramnnim

My position has been that energy is wasted with a flexy frame when pedaling out of the saddle, low rpm (60ish) and rocking the bike back and forth with your arms. Like...a less graceful version of Contador.

Based on what I saw on my Pioneer head unit while pedaling as described, my pedaling force is all straight down, from 2 oclock to 6. At 7 it is zero, and 8 is slight upward pressure. (meanwhile the other leg is pushing straight down) Which is why I disagree with you here, I think the flex at 6 oclock while pedaling as described is very similar to the flex of just pushing down while the bike is stationary.

If, when out of the saddle, the maximum downward leg force on the pedal is at 6 o'clock, then the maximum frame deflection, the maximum 'spring force' of the frame, and maximum strain energy in the frame, occurs at this point. If the downward leg force on the pedal decreases after this, then the larger 'spring force' of the frame acts to raise the leg, and the frame un-flexes. Since the frame is exerting a force (the frame 'spring force'), over a distance (the amount it un-flexes), then energy in the frame is being returned, and this energy is used to help raise the leg.

As long as the downward leg force decreases (which it must if the leg is to be raised), the frame must un-flex upward. The frame also has its 'spring force' that is exerted upward while un-flexing. Since energy is a force exerted over a distance (E = F x d), it must be transferring energy to the pedal as it un-flexes upward. Of do you think the Laws of Thermodynamics are wrong, and energy just disappears?

[kramnnim]

Not saying the energy disappears, I don't think it performs anything very useful. It takes away downward force and at best, helps to lift your leg a small amount. If the frame would not have flexed to start with, your leg would not need to be lifted that extra small amount.

[Kontact]

Originally Posted by kramnnim

Not saying the energy disappears, I don't think it performs anything very useful. It takes away downward force and at best, helps to lift your leg a small amount. If the frame would not have flexed to start with, your leg would not need to be lifted that extra small amount.

There is little point in talking about this if "lift your leg" is "not useful". That sort of 'logic' is going to make any sort of engineering explanation pointless. You simply don't believe it is possible and there is no point arguing with faith.

[kramnnim]

Yeah, it's quite clear after how many pages of the same thing going around in circles that we aren't getting anywhere.

[Mark McM]

Originally Posted by kramnnim

Not saying the energy disappears, I don't think it performs anything very useful. It takes away downward force and at best, helps to lift your leg a small amount. If the frame would not have flexed to start with, your leg would not need to be lifted that extra small amount.

I'm not sure what you mean by "takes away downward force". The downward leg force during the downstroke is the same, whether the frame flexes or not. In the case the frame flexes, the pedal travels downward further, but under the same force. The extra downward travel requires extra energy from the leg, which is transferred into the frame as strain energy. When the downward force from the leg decreases, the pedal (and/or BB) travels upward as the frame unflexes, and the energy is returned. If the leg down force is decreased before 6 o'clock, then the unflexing raises the BB more than the pedal, thus helping to rotate the crank, and some of the strain energy goes directly into the drivetrain. After 6 o'clock, the returned strain energy can't act to rotate the crank forward, but it does help lift the leg.

[kramnnim]

It my mind, it's like if you were trying to lift an object with a class 1 lever. You can only move the lever down so far before you hit the ground/floor. If the lever flexes, your downward force is wasted.

I don't think the upward lift at 6 oclock is useful because it is only giving back what it gave away. So you lost some downward force when you needed it, and don't get anything back.

[Mark McM]

Originally Posted by kramnnim

It my mind, it's like if you were trying to lift an object with a class 1 lever. You can only move the lever down so far before you hit the ground/floor. If the lever flexes, your downward force is wasted.

I don't think the upward lift at 6 oclock is useful because it is only giving back what it gave away. So you lost some downward force when you needed it, and don't get anything back.

Does the pedal hit ground when you step on it? No. If the lever doesn't hit the ground, does it transfer the same force, regardless of whether the lever flexes or not? Yes it does.

I think you need to go back and review the concepts of Force and Energy (Work).

[cachagua]

When the downward force from the leg decreases, the pedal (and/or BB) travels upward as the frame unflexes, and the energy is returned... The returned strain energy can't act to rotate the crank forward, but it does help lift the leg.

Lifts your leg, but can't rotate the crank forward. Exactly right.

[kramnnim]

Originally Posted by Mark McM

Does the pedal hit ground when you step on it? No. If the lever doesn't hit the ground, does it transfer the same force, regardless of whether the lever flexes or not? Yes it does.

I think you need to go back and review the concepts of Force and Energy (Work).

At 6 oclock, the downward force on the pedal can no longer provide rotational force...and additional downward force just flexes the frame, which doesn't benefit the rider.

[Kontact]

Originally Posted by kramnnim

At 6 oclock, the downward force on the pedal can no longer provide rotational force...and additional downward force just flexes the frame, which doesn't benefit the rider.

Nor does it actually happen. So why keep bringing it up as if it does? At 6 o'clock rainbows don't appear, either.

[kramnnim]

Originally Posted by Kontact

Nor does it actually happen. So why keep bringing it up as if it does? At 6 o'clock rainbows don't appear, either.

Nor does what actually happen?

[Kontact]

Originally Posted by kramnnim

Nor does what actually happen?

The frame doesn't flex down at 6 o'clock. That's not part of what happens during pedaling and has no place in the discussion.

There are all sorts of ways you could get a frame to flex by doing odd things to it, but those things would need to have something to do with pedaling to be relevant to a discussion about frame flex from pedaling.

[cachagua]

There are all sorts of ways you could get a frame to flex, but those would need to have something to do with pedaling to be relevant to a discussion about frame flex from pedaling...

Sounds like you're suggesting we leave frame flex induced by non-drive forces out of the discussion?

That's fine with me. Originally, we were trying to figure out whether or not frame flex subtracts from your efforts to move the bike forward -- and for purposes of that question, any other behavior of the bike, caused by any other force applied to it, is at best a distraction.

So where are we with that original question?

[Kontact]

Originally Posted by cachagua

Sounds like you're suggesting we leave frame flex induced by non-drive forces out of the discussion?

That's fine with me. Originally, we were trying to figure out whether or not frame flex subtracts from your efforts to move the bike forward -- and for purposes of that question, any other behavior of the bike, caused by any other force applied to it, is at best a distraction.

So where are we with that original question?

If it has anything to do with pedaling, that's what we're talking about. If you find a way to get the head tube to flex, probably not what we're talking about.


In terms of subtracting from forward movement, you should suggest a mechanism where a one way drivetrain (freewheel) can make energy go upstream. If not upstream, to where?

[cachagua]

In terms of subtracting from forward movement, you should suggest a mechanism where a one way drivetrain (freewheel) can make energy go upstream. If not upstream, to where?

I'm not sure I understand your question... nothing we've been talking about has to do with the freewheel. In fact we could be talking about track bikes, I think the phenomena we want to understand operate the same there. Didn't one of the videos earlier show a track bike?

"Upstream"? You mean, what I've been calling against the direction of drive torque?